Shielded spakk plug



Oct. 25, 1949. I R CHRISTIE 2,485,699

SHIELDED SPARK PLUG Filed Nov. 22, 1946 INVENTOR. file/=1 M BY I QNAQ Qa Patented Oct. 25, 1949 SHIELDED SPARK PLUG Robert K. Christie, Toledo,Ohio, assignor to Champion Spark Plug Company, Toledo, Ohio, acorporation of Delaware Application November 22, 1946, Serial No.711,497

2 Claims. (01. 123-169) This invention relates to a shielded spark plugof the type used in aircraft and is particularly directed to a manner ofassembling the component parts of such a spark plug.

It has heretofore been proposed to use a sleeve of malleable metal,either formed in situ or separately introduced and compressed, as asealing and holding medium between the core and shell of a spark plug.The bond formed by such a sleeve is quite satisfactory in use andbecause of its high thermal conductivity has proved useful ininstallations where it is important that heat pass rapidly away from thecore into the shell.

At high altitudes the tendency for a radio shielded spark plug to failis increased because of the formation of corona within the upper portionof the barrel to the extent that an electrically conductive path isformed between a part at high potential and some other plug part that isat a lower potential. To overcome this tendency to flash-over, manyexpedients have been tried including attempts to occlude air from thebarrel by completely filling this space with some flexible anddeformable insulating substance such as a silicone resin. While theintroduction of such material successfully prevents the ionization of acomplete discharge path it poses other problems. For example, thethermal expansion of the confined resin body is so great that pressuresexceeding 9000 pounds per square inch may be developed which are morethan suflicient to unseat a core held by a metal sleeve seal.

It is the primary object of the present invention to provide a shieldedspark plug in which the core is held in place so firmly that it cannotbe unseated by any pressure likely to be encountered in practice.

Another object of the invention is to provide a shielded spark plug inwhich the parts are assembled under pressure.

Other objects and advantages of the invention will become apparent fromthe following specification, reference being had to the accompanyingdrawings in which:

Figure 1 is a side elevation, with parts in section, of a spark plugembodying the present invention, the parts being shown during assembly;Fig. 2 is a top view of a locking sleeve; Fig. 3 is a section on line 33of Fig. 2; Fig. 4 is a view similar to Fig. 3 showing the sleeve in itscompressed form; and Fig. 5 is a central vertical section of a sparkplug insulator, and Fig. 6 is a section on line 6-6 of Fig. 1.

Referring to the drawings, a spark plug embodying the inventioncomprises generally a shell l0 threaded for engagement with an enginecylinder head, a core H within the shell and a shield barrel I2extending upwardly around the upper end of the core to form a recess inwhich electrical connection to the usual ignition lead is established.The shell [0 is provided with the usual hex portion [3 by which thespark plug is positioned in an engine.

A center electrode I4 is fixed in the center of the core and projectsoutwardly therefrom to form a spark discharge gap with one or more sideelectrodes I5.

The shield barrel l2 has, at its top, a threaded area I 6 for engagementwith an ignition harness, then an integral shoulder I1, next, a portionof uniform diameter, and at its lower end a reduced axial extension Ill.The outside diameter of the major portion of the barrel corresponds tothe inside diameter of the upper part of the shell I0 so that the twoparts fit into close telescoping engagement over a part of the length ofthe barrel and shell.

The shell I0 is further provided with an intermediate bore l9 thediameter of which corresponds to the outside diameter of the axialextension l8 of the barrel l2. The bore l9 terminates in a shoulder 20and receives a locking sleeve 2| between the end of extension l8 and theshoulder.

The locking sleeve 2| is preferably made of copper or other malleablemetal of high heat conductivity and as shown in Figs. 3 and 4 isinitially a uniform cylindrical member having an undercut or groove 22in one of its surfaces. In the form shown groove 22 is in the outsidesurface of the In assembling the parts the core II is slipped into thebarrel I2 and the locking ring 22 dropped into the bore l9 of the shell.The barrel and core are then inserted the proper distance so that, thecenter electrode I4 properly coincides with the side electrodes. Theparts are then put into a press one element of which may convenientlybear against the under side of the hex portion -l3 and the oppositeelement of which places a closing force on shoulder I! as indicated inFig. 1. It will be "seen that as the press is closed, all of the forceis exerted by the axial extension [8 of the barrel [2 to compress thelocking sleeve 2| against shoulder 20. Being of a malleable metal thesleeve 2| is deformed and flows into groove 24. This action isfacilitated by groove 22 in the locking sleeve since this latter rooveis initially opposite the groove in the core and deformation of thelocking sleeve will start at this point of smallest section.

The core is now held firmly in place with relation to the locking ring.While the press is still closed or after the operation has beentransferred to another press and the parts are under pressure, theassembly is completed by brazing or otherwise uniting the barrel l2 andthe shell [0. The braze is indicated at 30 in Fig. 1. It will be seenthat, in efiect, the plunger used to compress the locking ring remainsin place as a part of the assembly and is not withdrawn as is the usualpractice.

As previously stated, the groove 24 is made rounded so that the meta1with which it is filled does not exert a direct tensile stress on thecore when it expands under heat. The metal has a much greater rate ofthermal expansion than does the core. this type are ordinarilyfabricated is relatively weak in tension but very strong in compression.It will be apparent that forces tending to move the core axially ineither direction with respect to the shell put the core in compression.Cores locked in this manner have been found to resist pressures in theorder of 12,000 pounds per square inch in the barrel, being about thelimit of bursting strength of the barrel, without appreciable unseating.

The present invention thus provides a spark plug assembly which isreadily accomplished, strong in resistance to displacement of the partsThe material of which cores of and yet unlikely to be destroyed byheating and cooling cycles in use. While the invention has beendescribed in conjunction with a specific form and disposition of theparts, it should be expressly understood that it is capable of numerousmodifications and changes without departing from the spirit of theappended claims.

Having thus described my invention, what I claim as new and desire tosecure b United States Letters Patent is:

1. In a spark plug of the type described, a shell having an internalshoulder, a core received in said shell and having a circumferentialgroove, a locking sleeve for uniting said shell and core comprising aninitially substantially cylindrical body of malleable metal seated onsaid shoulder and having a circumferential weakening groove in one ofits surfaces adjacent the groove in said core, and means actuated byexternal pressure to deform said locking ring in such a manner that theshell and core are functionally united and a portion of the metal of thering flows into the groove in said core.

2. A spark plug as defined in claim 1 in which the circumferentialgroove in said core is so rounded in cross section that thermalexpansion of the locking sleeve material does not set up tensilestresses in the core material.

- ROBERT K. CHRISTIE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,356,104; Tognola Aug. 15, 19442,416,643 Rinehart Feb. 25, 1947

